1. Field of the Invention
The invention relates to seismic sensors and particularly to a device for detecting seismic signals along mutually orthogonal axes.
2. Discussion of the Related Art
In seismic exploration, a seismic source such as a vibrator or an explosive charge, introduce a seismic signal into the earth. The seismic signal propagates through the earth from the point of introduction as an expanding spherical wave front. As the wave front impinges upon textural or structural changes in the subsurface, a portion of the wave front may be reflected back to the surface. The reflected seismic signals that arrive at the surface may be detected by seismic sensors such as geophones deposited on the land or hydrophones deployed in water-covered environments.
The seismic signals originating from the source and reflected back to the surface typically include several types of seismic waves, each having distinguishing characteristics from the others. The types if seismic waves include compressional and shear waves, generally referred to as body waves, and two types of surface waves known as Raleigh waves and Love waves. Compressional waves and shear waves, commonly referred to as primary (P) and secondary (S) waves respectively, are of particular interest to exploration geophysicists, because they propagate at different velocities, and because they propagate to great depths unlike the surface waves.
Compressional and shear waves each have a distinct particle motion. In compressional waves, the particle motion of a propagating wave consists of alternating condensations and rarefractions during which adjacent particles of the propagating medium move closer together and further apart. The motion of the particles in a compressional wave is therefore always in the direction of wave propagation. In a shear wave, particle motion consists of undulations parallel to the wave front where the particle motion is always perpendicular to the direction of wave propagation. If during the propagation of a shear wave, the particles all undulate in parallel lines, the wave is said to be polarized in a direction of the undulations. A horizontallytraveling shear wave polarized so that the particle motion is all vertical is designated as an SV or vertical shear wave; when its particle motion is all horizontal, it is called an SH or horizontal shear wave. Shear waves may be polarized in planes other than the vertical and the horizontal but for the purposes of study, their components may be resolved in horizontal and vertical planes.
Because of the different particle motion and propagation velocities of the two wave types, compressional wave and shear waves are important in determining the characteristics of the subsurface. The two-wave types are used to "fingerprint" the propagation characteristics of the subsurface formations. These characteristics of the subsurface may be measured by placing sensors along the surface of the earth or by placing sensors at different depths in a bore hole.
In the past, detection of the two body waves has been accomplished by using conventional geophones. The geophones are placed on the ground with their single axis of sensitivity oriented either horizontally to detect shear waves, or vertically to detect the compressional waves. Each sensor had to be properly placed on the ground to assure proper orientation with respect to the propagation direction of the desired signal to be detected. This was particularly true for sensors used to detect frequencies 14 Hertz (Hz) or less. The low frequency sensors would become inoperable at tilt angles of 5 degrees or more.
In later detectors, three sensors were mounted to a single chassis such as that the axis of sensitivity for the sensors were fixed 90 degrees to each other. Thus, one unit consisting of three sensors could be used to detect both compressional waves and shear waves. The same problem of orientation existed in this sensor as it did for a single sensor. The sensors must be properly oriented so that the axes were both vertical and horizontal, so that each wave type could be detected.
Similar problems existed in vertical seismic arrays where the sensor was deployed in the borehole. Boreholes are not perfectly cylindrical. There are irregularities in the borehole diameter as well as inclination. Often sensors disposed in boreholes assumed orientations other than vertical or horizontal when wedged tightly against the side of the borehole. To solve the problem of tilted sensors in a borehole, the sensors were hung in the casing on a trunnion. One sensor may have been mounted vertically and two sensors may have been mounted horizontally, but at 90 degrees to each other. As the sensor case became inclined when forced against the borehole wall, the trunnioned sensors where supposed to hang vertically in the proper orientation so as to detect both the compressional and shear waves. A major disadvantage in this arrangement was that the horizontal sensors did not remain orthogonal to each other when tilted. The horizontal sensors would be orthogonal with the housing oriented vertically, but as it was inclined, the two horizontal sensors would assume orientations where their axes were not 90 degrees to each other. If the housing were to be oriented horizontally and the sensors allowed to pivot freely, they could be essentially parallel to each other, and thus detect the same signals.
Other attempts to resolve the problem of maintaining sensor orthogonality resulted in the use of gimbals. Three geophones fixed on a single chassis were mounted on a dual-axis gimbal. This resulted in a gimballed-transducer assembly having a diameter too large for use as a downhole tool. Additionally, the orientation of the chassis became indeterminate when tilted too far along the pivot axis of the outer gimbal. The primary disadvantage in using the single-gimbal geophone assembly was size.
FIGS. A-C show a later modification which included separately mounting each sensor on a gimbal. The individual mounting of the sensors allowed a reduction of the tool diameter, but had the same problem in maintaining the orthogonality of the sensor axes. As the tool was tilted, the gimbals pivoted so that the axes of the two horizontal sensors, originally orthogonal to each other in a first position, became substantially parallel to each other in a second tilted position. This resulted from the rotational offset of the outer pivot axis of one gimbal 90 degrees to the outer pivot axis of the other gimbal. As the gimbals pivoted, the inner pivot axes became parallel and thus, so did the axis of sensitivity of the sensors. Another major disadvantage in the individually-gimbaled geophone is indeterminant positioning of the geophones when tilted beyond some critical angle in the plane including the outer pivot axis. When an outer pivot axis is tilted on end, the restoring or gravitational force required to stabilize the gimbal cannot pivot about the axis, thus, the geophone is allowed to wander about the inclined pivot axis.